Alternative to python list methods for bpy_prop_collection instances? Most interested in sorting
The blender python API is an API over Blender's internal structure. These structures are designed for performance, hence it cannot be influenced directly.

But the developers have made it quite easy, as you can easily create a local list to do more python stuff. But it is not easy to transfer the results back into blender's database.

spline_collection = list(obj.data.splines)
spline_collection.sort(key = lambda x: get_spline_length(x), reverse = True)


Ps same reason why deepcopy does not work as these objects are not fully python. Most objects have a .copy method you can use.